This invention relates to a plasma treatment system, particularly to a plasma treatment system suitable for suppressing irradiation damage of a substrate by charged particles in plasma and for forming an oxide film or nitride film containing no hydrogen.
In the production of IC or LSI, insulator films of SiO.sub.2, Si.sub.3 N.sub.4, etc. are much used as an interlevel insulator film, insulator film for charge storage capacitor, or a gate insulator film, and these insulator films are produced using a high temperature process, for example, by CVD (chemical vapor deposition), by thermal oxidation, etc. However, the process requires a heat treatment at a temperature as high as 1,000.degree. C., and thus is less applicable to a step where high temperature heat treatment is undesired, such as the final step in the LSI production, or inapplicable to production of thin film transistors, etc. using low melting point substrates.
To overcome the disadvantages of the high temperature CVD process, a plasma CVD utilizing plasma of a reactive gas, which will be hereinafter referred to as PCVD, has been recently regarded as important. According to PCVD, a reactive gas containing Si as a component atom such as monosilane (SiH.sub.4), and an oxygen gas or other gas such as a nitrogen gas or an ammonia gas are supplied to a chamber made of stainless steel, etc. at the same time, and a DC or AC high electric field is applied thereto from an external source while maintaining these gases in a dilute state, thereby generating a plasma and depositing SiO.sub.2, Si.sub.3 N.sub.4, etc. on a substrate.
In FIG. 1 a parallel plate-type, PCVD apparatus is schematically shown. A substrate to be treated, that is, wafer 11, is provided on one of two counterposed electrodes 12, and a DC or AC high voltage is applied between the two electrodes 12 from an electric source 13 to subject a dilute reactive gas to electric discharge. The reactive gas is decomposed by the electric discharge, and reacts with activated oxygen, nitrogen, etc. existing at the same time to form an insulator film on a substrate, where numeral 14 shows a matching box, 15 an earth shield, 16 an evacuation system, 17 a gas introduction system, and 18 a vacuum chamber. However, a high voltage is also applied to the substrate according to this process, and thus charged particles on a high kinetic energy level collide with the substrate to cause so-called irradiation damage. To overcome this disadvantage, it has been proposed to use a microwave PCVD apparatus, etc. utilizing a microwave discharge of 2.45 GHz as one of electrodeless discharges to restrict the substrate bias only to a floating potential of 10-20 volts.
According to these PCVD processes, it is possible to form an insulator film around room temperature, but the thus formed films have such a serious disadvantage that a large amount of hydrogen is contained in the films. The hydrogen is produced as a by-product from the decomposition of SiH.sub.4 or NH.sub.3, and easily discharged from the films at a high temperature CVD, whereas the hydrogen remains in the films in the case of PCVD because of the low substrate temperature. The hydrogen in the films acts as a trap source for electrons injected into the insulator films, causing fluctuations in the flat-band voltage or the threshold voltage. Particularly with recent advance in miniaturization of devices and thinner insulating films, electrons are more readily injected, and thus the hydrogen in the insulating film has been a serious problem in the long-term reliability of devices.